1. Field of the Invention
The present invention relates to a switching mode power supply (SMPS). More specifically, the present invention relates to a switching mode power supply for compensating for variations of maximum output power caused by variations of an input power.
2. Description of the Related Art
A switching mode power supply (SMPS) is a device for converting a direct current (DC) supply voltage into a DC output voltage. The DC output voltage can be greater or less than the DC supply voltage. The switching mode power supply is mainly used for battery-based power supplies installed in electronic devices such as mobile phones and laptop computers. Such electronic devices are operable in a normal operation mode with a relatively large amount of power consumption and a standby operation mode with a relatively small amount of power consumption.
However, conventional switching mode power supplies generate unstable output power because of propagation delays generated by elements of the switching mode power supply when the input voltage is varied. Propagation delays include internal propagation delays of a controller and turned-off delay times of a switch.
FIG. 1 shows the current flowing to a main switch coupled to the primary side of a general switching mode power supply in the ideal case of no propagation delay. In typical configurations the main switch is a MOSFET and the current is the drain current of a MOSFET. In such ideal configurations the peak current flowing to the main switch is controlled to be constant according to feedback information (i1P) at an output terminal even when the input voltage varies. This feedback ensures that the maximum power transmitted to the output terminal is controlled to be constant.
FIG. 2 shows the main switch current on the primary side of a general switching mode power supply with propagation delay. The peak currents of the main switch are different depending on the input voltage when a propagation delay of Δt is generated in the SMPS. Without a propagation delay for low and high Vin input voltages the times to reach the i1P peak current are t1 and t2, respectively. With a propagation delay the main switch current continues rising after t1 or t2, and therefore the peak current varies depending on the input voltage. In detail, since the main switch current's gradient di/dt=Vin1/L1 is small for low a input voltage, and the gradient di/dt=Vin2/L1 is large for a high input voltage, the peak currents i1 and i2, reached after the propagation delay Δt, are different.
The difference of the peak currents on the primary side causes the maximum output power at the output terminal depend on the input power because of the presence of the propagation delay of the SMPS.